Isocyanate mixtures having-CD and/or UI groups may be produced simply with the highly effective catalysts from the phospholine series, in particular the phospholine oxide series, by the process according to U.S. Pat. No. 2,853,473, U.S. Pat. No. 6,120,699 and EP-A-515 933.
The high catalytic activity of phospholine catalysts, in particular phospholine oxide catalysts, is desired on the one hand to activate the carbodiimidization reaction under mild temperature conditions, but on the other hand there has hitherto been no known process for ensuring that phospholine or phospholine oxide catalysis is effectively stopped without restriction. The carbodiimidized isocyanates have a tendency to post-react, i.e. they gasify as a result of CO2 formation. This then leads, particularly at higher temperatures, to a build-up of pressure for example in the storage containers.
There has been no shortage of attempts to find an effective means of stopping phospholine catalysis. Such stoppers are mentioned e.g. in patent specifications DE-A-25 37 685, EP-A-515 933, EP-A-609 698 and U.S. Pat. No. 6,120,699 and comprise e.g. acids, acid chlorides, chloroformates, silylized acids and halides of elements of the main group. Stopping the catalyst with acids which, e.g. may also be present as acid chlorides, is not sufficiently effective.
According to the disclosure of EP-A-5 15 933, CD/UI-containing isocyanate mixtures produced by phospholine catalysis are stopped with at least an equimolar quantity, preferably a 1-2 fold molar quantity in relation to the catalyst used of e.g. trimethylsilyl trifluoromethane sulfonate (TMST). In practice however, it has proved that CD/UI-containing isocyanates produced in this way are not universally suitable for the production of pre-polymers, i.e. reaction products of these CD/UI-containing isocyanates with polyols. The reaction products produced accordingly from polyols and the CD/UI-modified isocyanates have a tendency to gasify, which can lead to a build-up of pressure in transport containers or to foaming during handling of these products.
This problem can be avoided by using the silylized acid used according to EP-A-515 933 in higher molar equivalents (e.g. 5:1-10:1 in relation to the catalyst) to stop the phospholine catalyst. In practice however, it is then found that the CD/UI-modified isocyanates obtained have a considerably poorer color index. This therefore also applies to the pre-polymers produced from them.
This applies also if the phospholine catalyst is stopped with acids of the trifluoromethanesulfonic acid type according to U.S. Pat. No. 6,120,699. Pre-polymers produced from these also have a considerably increased color index.